1. Field of the Invention
The present invention relates to an image processing apparatus, a compression method and an extension method.
2. Description of Related Art
In a copying machine or a printer, and the like, when an image is saved in a memory, compression processing is generally performed in order to reduce data amount. As a compression method thereof, JPEG (Joint Photographic Expert Group) or BTC (Block Truncation Coding), and the like are generally known. However, in a case where a region where a high resolution is required such as in the case of texts or graphics, and a region where a high gradation is required more than a high resolution such as in the case of images, are mixed in one image, the image quality is likely to be deteriorated when the entire image is compressed by JPEG because the texts and the graphics cannot be maintained with high resolution. As described above, the resolution and the gradation are incompatible when compression processing is performed by a single compression method, when the image has mixed regions in which an attribute of the image is different from one another.
Thus, a method is disclosed in which one image is divided into blocks to be performed with compression processing, and multiple coding processing such as JPEG and dual coding processing such as MH coding method, are switch from one another in a unit of blocks so as to be applied (see for example, Japanese Patent Application Laid-open Publication No. 8-9167).
However, there still is a case in which an image of texts or graphics and an image of photographic images are mixed in one block. For example, cases where texts written on a black board are included in a picture, or texts are included on a T-shirt worn by a person in a picture when the person is photographed. When the compression processing by JPEG is performed for the block as an image of a picture, the images of texts or graphics included in the image of the picture is altogether performed with JPEG, thus resulting in the loss of resolution.